Re: [Qemu-devel] [PATCH v4 11/11] nbd-server: Allow node name for nbd-server-add

[Eric Blake]

On 07/15/2016 07:36 AM, Max Reitz wrote:
> On 14.07.2016 23:36, Eric Blake wrote:
>> On 07/14/2016 07:28 AM, Kevin Wolf wrote:
>>> There is no reason why an NBD server couldn't be started for any node,
>>> even if it's not on the top level. This converts nbd-server-add to
>>> accept a node-name.
>>>

>> Do we want to do any sanity checking that writing should only be
>> permitted on a root, and that when using a node name that is not a root
>> that writable must be false so as not to negatively change the BDS out
>> of under the feet of the other root?  Do op-blockers already cover that?
> 
> Well, one could argue that it's possible to create an NBD server on a
> non-root node today anyway, since creating BBs is not restricted to root
> nodes:
> 
> blockdev-add(id=foo, other arguments...)
> blockdev-add(id=bar, backing=foo, other arguments...)
> 
> And then you can create an NBD server on bar. I agree that this is not
> how it should be, though. However, I think that the fact that you need
> to specify a BB name for now deters people from doing stuff like that.
> If you can specify a node name, people will think it's completely fine
> to do so.

Creating a server on bar doesn't change the contents of foo, so I see
that as safe (foo can still be in use by other chains, and the server on
bar won't invalidate those chains).

> 
> Also note that only allowing NBD servers to be created on a root node
> doesn't really help you:
> 
> blockdev-add(node-name=foo, ...)
> nbd-server-add(device=foo)
> blockdev-add(id=bar, backing=foo, ...)

But THAT is indeed unsafe, if the server allows writes, because now the
contents of bar are at risk of being silently changed by any edits made
to foo.

So the real restriction we want is that if foo is owned by a read-write
BB (the NBD server in this case), then creating another BDS bar that
uses foo as a backing is undesirable.

> 
> So, yeah, I think we just need the new op-blockers for this, I don't
> think the current op blockers cover this.

I'm not sure either, which is why we're discussing it on list to make
sure we think about the restrictions and their implications.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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